The present invention relates to an apparatus for reproducing an information signal recorded on an optical disc along spiral or concentric tracks comprising a light source for producing a light beam, an optical system for focusing the light beam onto the optical disc as a light spot, a light detector for receiving the light beam modulated in accordance with the information signal recorded on the disc to produce a reproduced signal, means for rotating the disc at a given revolving speed, and means for returning the light spot on the disc by one track pitch after every revolution of the disc so as to effect a special mode reproduction such as slow motion and still picture mode reproduction.
In a video disc, a video signal is so recorded that successive frame signals are recorded in successive tracks. In order to effect a still picture mode reproduction, the beam spot must be quickly moved backward by one track pitch in a stepwise manner in synchronism with a frame synchronizing signal derived from a reproduced video signal so as to cause the beam spot to trace the same track repeatedly. For effecting the still picture reproduction accurately, the beam spot should be returned precisely by one track pitch. This may be effected by rotating a movable mirror by a small angle or by shifting an objective lens in a tracking direction substantially perpendicular to an optical aixs of the lens as well as to a track direction. However, in practice, it is very difficult to return the beam spot accurately by one track pitch by means of an electro-mechanical mechanism for driving the mirror or lens. That is to say although a given control amount required for moving the beam spot by one track pitch has been previously set, an actual amount of displacement of the beam spot might be fluctuated due to a tracking control for moving the beam spot on the disc in its radial direction so as to cause the beam spot to trace the track accurately. In general, the disc has an eccentricity with respect to a spindle for rotating the disc and thus, a tracking error is produced in synchronism with the rotation of the disc. Therefore, in order to compensate the tracking error, the beam spot is moved at a rhythm of the revolving period of the disc. Further, when the beam spot is incident upon a portion of the disc at which the frame synchronizing signal is recorded, the beam spot may be moved either outwardly or inwardly in the disc radial direction in accordance with a tracking error which has been just detected at this moment. That is to say, the periodic phase of the eccentricity and the periodic phase of the frame synchronizing signal may be mutually varied arbitrarily depending upon a position or situation of the disc placed on a turn table. Therefore, even if a precisely determined control signal is supplied to a driving mechanism for moving the beam spot by one track pitch, the beam spot is not shifted accurately by one track pitch. It is assumed that the still picture reproduction mode is achieved by moving the beam spot inwardly in the disc radial direction. When the beam spot is moved also inwardly so as to correct the tracking error at the time of displacement of the beam spot, the beam spot is moved inwardly by more than one track pitch. Contrary to this, when the beam spot is moved outwardly under the control of the tracking servo, the beam spot could not reach a given track. It is apparent that the tracking error may occur entirely independently of the postion of the frame synchronizing signal on the disc and thus, even if the amount of the control signal for returning the beam spot is precisely determined, it is impossible to move the beam spot accurately by one track pitch.